RaopX – stream music to an AirPort Express, Mac-specific, functions as a virtual output device.

raop-play – stream music to an AirPort Express. I couldn’t get this to work though (error 453, which apparently means the APX is already in use, which it wasn’t; nor was it able to connect to shairport).

pulseaudio-module-raop – Ubuntu module; virtual PulseAudio device to stream to an AirPort Express (stutters like crazy, although it appears to depend on the application. Youtube in the Browser works fine).

pulseaudio-raop2 – there apparently exist two vesions of the AirTunes protocol, and this might work better (not tried).